The Western saddles; Billy Cook, Circle Y , Tucker & used western trail saddles have on line tools to measure your horse for a saddle. I think Tucker also offer the same info on line.
Go to the hardware store and get about 4’ of 12 or 14 gauge house wire (the kind electricians use to wire a house) and cut it in half. This was written for Western saddles but the principle is the same for English or Australian.
Take 2’ and shape it over your horse’s withers. Take the other 2’ and shape it over the center of his back.
Carefully take the wires and trace the inside of the wire on a heavy piece of cardboard (or poster board if you have it). Cut out the cardboard shapes and take them to the saddle shop to fit against some saddles.
This part is Western: Keep in mind that QH bars in one saddle may not be the same in another brand. There are no standards for saddle trees so each manufacturer has his own idea what dimensions make the designations.
As for you, your bum should not be squished against the cantle, it should have a little room at the top and there should be about 4" of space between your tummy and the swell.